1. Field of the Invention
The present invention relates to the field of frequency planning for wireless networks.
2. Description of the Related Art
As wireless subscribers travel, they are switched between different transmitter, or cell, sites. Each site may be divided into sectors, with each sector served by one or more base stations located at the transmitter site. A base station, or base transceiver station, comprises an antenna and a radio transceiver at the cell site. In order to accommodate as many users as possible, the base station is not in constant contact with each wireless device operating in its range. Instead, when a wireless device enters the coverage area of a cell, it contacts the server base station through a control channel. This control channel carries information between the wireless device and the server base station necessary for the wireless device to operate properly with the server base station. When a user initiates a wireless operation, the wireless device instructs the server base station, through the control channel, that the device is attempting an operation. The server base station then switches the wireless device to a traffic channel to conduct the operation.
As the wireless subscriber continues to travel, the wireless device must switch from one cell to another. This process is known as a handoff. To facilitate the handoff process, the wireless device constantly measures the control channel signal strength of the server base station, and the signal strength of the signals from base stations serving adjacent sectors or located at neighboring towers, to determine which one will provide the best service. The wireless device transmits these signal strength measurements back to the server base station, which stores the measurements to compute an average over a short period of time. In a GSM system, the data is transmitted once every 480 microseconds and in TDMA systems once per second. As these data are transmitted so frequently and the wireless subscriber may be moving rapidly, any data over ten seconds old is not useful for purposes of evaluating the need for a handoff and is constantly purged from the server base station shortly after receipt.
When neighboring sites or sectors are transmitting the same frequency (i.e., are co-channels), they may interfere with each other. To mitigate this co-channel interference, cellular providers institute frequency planning. The carrier-to-interference (C/I) ratio is a measure of the strength of the desired signal relative to that of interference signals.
To track the level of interference, it is standard to construct a matrix of C/I values for neighboring base stations. The frequency planners are then able to use the data in this matrix to better adjust for interfering frequencies. Several prior art methods of developing this matrix are currently in use.
One method for creating the C/I matrix is often termed the “Listening only Control Channel” (the “LICC” or “Ericsson”) method. This method entails measuring the control channel signal strength of the site in question. A LICC capability must be added for each site sector (using otherwise valuable bandwidth). This method involves measuring the signal strength of the uplink (the signal from the wireless device to the tower), and during the data collection period, the two traffic channels associated with the control channel must be blocked. When a user originates a call, a request is sent over the control channel to its server base station. It is the strength of this uplink signal that is measured.
There are a number of drawbacks to this method: (1) the only measurement made is during the initial communication with the base station and because the data points collected are limited to those associated with originations, the geographical scope is limited; (2) the transceiver uses valuable bandwidth that thus cannot be used for normal communications; (3) cells for which most of the traffic involves handoffs do not provide enough data points for the C/I matrix; and (4) the only measurement taken is on the uplink and therefore this approach does not actually measure the signal strength on the downlink, so that, as indicated above, only a limited number of data points are collected as compared to the average call length.
A second method for collecting interference data is the “Drive Test” method. In this method, a color code identifies each base station by frequency. A technician travels to various geographical locations and measures the signal strength at that location. The digital verification color code identifies the base station transmitting each signal. The technician measures all of the signals at each location and the strongest signal should indicate the server station. Therefore, the matrix must be manually generated by entering the data collected for each station by hand.
The first drawback of the “Drive Test” is that this method is geographically limited. It will not be possible to take measurements from within many buildings or on side streets, so the areas sampled will be limited. Power control is in the downlink (tower to wireless device) direction, and the station will dynamically adjust power as necessary to ensure transmittal. Therefore, the measurement may be of an intentionally low power signal. This test is expensive to implement because someone must be paid for the time of driving between sites to take measurements and entering the data. Also, changes in the topography and signal propagation resulting from new buildings and other structures require taking new measurements. The color code system only functions if there is a moderate level of interference. If two signals measured are both strong, then it will be difficult to decode the color code, and the technician must manually turn the signals on and off at each base station to test them and to determine which is likely causing the interference.
A third method is called the “Predictive Method” and uses propagation models. Because each signal degrades as it propagates through the air, computer models may be used to determine where the signals may interfere. However, this method also suffers from several drawbacks. First, there is a high degree of error because there are no actual measurements. Second, the models do not take into account differences in terrain or buildings. Third, any changes in the system require a new evaluation. Therefore, the model is inherently conservative in order to take into account the practical differences.
Because each of the three methods has a high cost in labor, or equipment, or both, there is a need for a system to inexpensively collect and process the necessary data for a C/I matrix.
Additionally, each of the prior methods accurately measures or predicts only that interference relating to the geographical features existing at the time the measurements are taken. It is desirable to have an automatic process that continuously collects new data as new buildings, roads and highways are constructed. To the extent that the prior art methods attempt to collect and measure actual data, these methods require the use of extra equipment or the dedication of valuable bandwidth to the measuring process, rather than keeping the bandwidth available to service customers.
Further, none of the prior methods result in measurements that accurately reflect the interference within the system during operation because the data is collected during a very small amount of time as compared to the average length of a call, is limited in geographic scope because a technician is not able to access every possible location to take measurements, or has a high error rate because the method is based on computer models, not actual circumstances.
Therefore, a system that uses current data and continuously monitors all calls from any geographic location and collects data during the full duration of these calls without using additional bandwidth would be very desirable and useful. Desirably, such a system would collect accurate and complete real-time, actual-use data to create a more accurate and useful C/I matrix and would thus enable the provider to better plan its frequency usage, thereby more effectively using available frequencies and better serving its customers.